The pronoun nemo is usually said to have only nominative, accusative and dative forms (nemo, neminem, nemini). The other forms, including plural, are easy to form, since nemo seems to come from ne+homo and homo is easily declined:
nemo *nemines neminem *nemines *neminis *neminum nemini *neminibus *nemine *neminibus
The genitive neminis and ablative nemine are found in Plautus and also in later writers. These forms are rare and (almost, at least) absent in the classical era. I do not understand why these two forms would be considered bad style. The plurals are nowhere to be found.
Do we know what happened to the missing forms? That is, do we know why and how they were left out? As a bonus question, have the plurals ever been used?
The plural forms are admittedly a little unnatural, but I can imagine situations where I would like to use them. If I give a lecture and expect to have a large audience but not a single person shows up, I would be tempted to say nemines aderant. The plural is particularly attracting for events where a single person could not meaningfully attend — a priest cannot wed a single person, for example. Alternatively, if I speak with some utterly unimportant people1, I might say cum neminibus locutus sum. Whether or not you like the examples, I hope you can agree that it is not completely unreasonable to use the plural forms sometimes.
1 I don't classify anyone I know in this category; this might be natural for passers-by that I have no relation to whatsoever. Certainly no user of Latin.SE qualifies as utterly unimportant to me.